Rare earth permanent magnets have found particular utility in many commercial applications, including electric motors, NMR scanners, and the like. The advantage of permanent magnets in these applications is their ability to exhibit high level, constant magnetic fluxes without applying an external magnetic field or electrical current. Early such magnets include samarium-cobalt rare earth intermetallic compounds, such as SmCO.sub.5 and Sm.sub.2 CO.sub.17. More recently, iron-neodymium-boron and other rare earth-iron/cobalt-based intermetallics have been investigated due to their superior magnetic properties. Magnets made from some of these rare earth-iron/cobalt-based intermetallics (e.g., Nd.sub.2 Fe.sub.14 B.sub.1) are known to require the presence of some (i.e , about 2-5%) elemental rare earth for optimal properties. Consequently, it is imperative to maintain a higher than stoichiometric level (i.e., for the intermetallic) of the rare earth in the final product.
A known method of making samarium-cobalt and other rare earth-iron/cobalt-based magnetic powders is by the so-called "reduction-diffusion" process wherein rare earth compounds such as rare earth oxides, chlorides or fluorides are reduced with a stoichiometric excess (i.e., about 30% excess) of elemental calcium (initially as calcium or calcium hydride) in the presence of the iron and/or cobalt (or Ca-reducible compounds thereof) and the resulting rare earth diffused into the iron/cobalt at elevated temperatures. Subsequent processing produces a Ca-free metallic powder which is ground into particles small enough (i.e., about 1-5 microns) to contain a preferred magnetic domain. The particles are then aligned in a magnetic field and pressed to form a compact and prevent relative motion of the particles. The compact is then sintered, heat treated and magnetized in the prealigned direction.
In conventional samarium-cobalt reduction-diffusion processes, samarium oxide, calcium and/or calcium hydride and cobalt are heated together to reduce the samarium oxide and diffuse the samarium into the cobalt. The resulting mass of rare earth-intermetallic, calcium oxide and unreacted calcium is hydrated with water to alkalize the Ca/CaO and form calcium hydroxide therefrom. The heavier intermetallic settles out while dissolved and undissolved Ca(OH).sub.2 floating in the supernatant liquid are removed by decantation. Thereafter, the intermetallic is washed with a weak acid (e.g., acetic acid) or an acidic solution of NH.sub.4 Cl to remove any residual Ca(OH).sub.2 therefrom.
The aforesaid process for making samarium-cobalt magnetics powders has been proposed for making other rare earth-ferromagnetic metal alloy powders. The Ca(OH).sub.2 -removal process used in the samarium-cobalt process, however, has not proved effective to produce rare earth intermetallics which require a second, elemental rare earth phase for optimal magnetics (e.g., Nd.sub.2 Fe.sub.14 B.sub.1 and Nd). In this regard, removal of the calcium hydroxide from Nd and Nd.sub.2 Fe.sub.14 B.sub.1 mixtures by washing with acetic acid serves only to dissolve the highly reactive elemental rare earth phase and thereby leave the resulting mixture too lean with respect to elemental rare earth content for optimal magnetic properties.
U.S. Patent Sharma No. 4,917,724 issued Apr. 17, 1990, and assigned to the assignee of the present invention, strips the Ca(OH).sub.2 from particle mixtures of a rare earth and its alloys by washing with an ammoniacal alkaline solution containing a reagent which forms a calcium salt which is soluble in alkaline solution. Sharma maintains the pH of the solution above 9.0 to prevent dissolution of elemental Nd when the soluble calcium salts are formed. However, the presence of ammonia is undesirable in a manufacturing plant and requires costly ventilation and air treatment facilities. This disadvantage could be eliminated if an NH.sub.3 -free system could be devised for stripping the reacted and unreacted calcium from the mixture without appreciably dissolving the elemental rare earth component of the mix. Moreover, economics could be achieved by elimination of the hydration step following the reaction-diffusion step.
Accordingly, it is the primary object of the present invention to provide a substantially NH.sub.3 -free process for stripping calcium and its reaction products from rare earth reduction-diffusion reaction products having an elemental rare earth component (preferably Nd+Nd.sub.2 Fe.sub.14 B.sub.1) without appreciable loss of the rare earth component. It is a further object of the present invention to provide such a process which does not require a hydration step following the reduction-diffusion step to alkalize the Ca/CaO present in the product produced by the reduction-diffusion reaction. These and other objects and advantages of the present invention will become more readily apparent from the detailed description thereof which follows.